The rapidly diminishing conventional oil reserves and the ever-increasing demand for liquid fuel (diesel and jet fuel in particular) have stimulated a surge in the global pursuit for non-conventional liquid fuels in the desired C7-C19 range. Currently, Fischer-Tropsch catalysis based on syngas oligomerization is a major contributor to non-conventional liquid fuels which can lead to an increase in the production of light alkanes. Light alkanes, C1-C5, are also highly abundant in current world oil and gas reserves. High accumulation of undesirable light alkanes calls for the formulation of efficient methods to upgrade low molecular weight hydrocarbons to heavier molecular weight hydrocarbons in the desirable range of C7-C19. One way to transform these abundant alkanes to versatile olefins is via cracking in refining industries. Catalytic cracking processes are often performed at very high temperatures (>500° C.) and thus are limited by reduced energy efficiency and poor product selectivity. In recent years much research has focused on transforming unproductive alkanes to highly versatile olefins with better selectivity and at lower operating temperatures. Transformation through alkanes dehydrogenation has shown some success.
Several homogeneous catalytic systems have been shown to accomplish alkane dehydrogenation at lower temperatures (<250° C.). Recent years have seen progress in dehydrogenation of alkanes and alkyl groups under homogeneous conditions using organometallic systems. A significant milestone in this regard has been the design and use of pincer-ligated iridium complexes for alkane dehydrogenation. The first report of alkane dehydrogenation came from Kaska and Jensen using (tBu4PCP)IrHn (1-Hn; R4PCP=κ3-C6H3-2,6-(CH2PR2)2; n=2 or 4). (See Gupta, M., et al., Chem. Commun. 1996, 2083). The Goldman group subsequently reported the greater catalytic activity of the less crowded iPr4PCP analogue. (See Liu, F., et al., Chem. Commun. 1999, 655). This has been followed by reports of numerous catalytically active variants with the (PCP)Ir motif, including other bis-phosphines, bis-phosphinites (POCOP), hybrid phosphine-phosphinites (PCOP), arsines (AsOCOAs), hybrid phosphine-thiophosphinites (PSCOP), and hybrid amine-phosphinites (NCOP). These complexes have also been employed for numerous other catalytic transformations of hydrocarbons, including alkane metathesis, alkyl group metathesis, dehydroaromatization, alkane-alkene coupling reactions, borylation of alkanes, and the dehydrogenation of several non-alkane substrates. More recently, several other pincer motifs have been explored for alkane dehydrogenation, such as (PBP)Ir, (CCC)Ir, (PCP)Ru, (PCP)Os, and (NCN)Ir.
Starting with Crabtree's report (Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1979, 101, 7738), 3,3-dimethyl-1-butene (TBE) has been used as an effective hydrogen acceptor for alkane transfer dehydrogenation. This is mainly due to the fact that TBE is not only resistant to double-bond isomerization, but also weakly coordinating, thus minimizing inhibition of catalysis. Norbornene (NBE) is also effective, presumably for similar reasons. Crabtree and co-workers had noted that the less bulky ethylene deactivated the catalysts via formation of stable complexes. However on a large scale, the use of smaller olefins, particularly ethylene and propene, would be much more practical. Recent years have seen a surge in the number of reports that describe the use of propene and ethylene as an acceptor for a variety of reactions such as dehydrogenation, dehydroaromatization, synthesis of piperylene, toluene, and p-xylene.
Of particular interest is the dehydrogenation of light alkanes, such as butane and pentane. The resulting primary (olefin) and secondary (dienes) dehydrogenation products are versatile and could potentially be dimerized (or cross-dimerized) to give alkanes of molecular weight more suitable for fuel, for example, in the C7-C19 range.
Remarkably high turnover rates in the molecular pincer-iridium catalyzed gas-solid phase transfer-dehydrogenation of light alkanes (which are generally undesirable as transportation fuel components) using economical gaseous olefins such as propene and ethylene have been recently reported. (Kumar, A., et al., J. Am. Chem. Soc. 2015, 137, 9894). The resulting light olefins and dienes have potential applications as precursors for fuel chemicals. In contrast to non-molecular solid-phase systems, these molecular solid-phase systems retained their characteristic behavior in solution and are selective for the formation of α-olefins resulting in yields of α-olefin much greater than have been previously obtained from homogeneous solution phase systems. The gas-solid phase transfer-dehydrogenation can be considered as unsupported heterogeneous reaction as it occurs by coating the molecular pincer-iridium catalyst on the walls of glass. This is thus different from earlier reports where the pincer catalyst is supported on solid supports via polar anchoring groups. (Huang, Z., et al., Adv. Synth. Catal. 2009, 351, 188 and Huang, Z., et al., Adv. Synth. Catal. 2010, 352, 125).
The industry is always searching for improved processes for producing valuable hydrocarbons in the liquid fuels range of C7-C19. Effective and efficient processes of transforming light alkanes of minimal value to valuable fuels hydrocarbons in the C7-C19 range are of particular interest. One object of the present invention is to provide such a process.